Field of the Invention
The invention concerns a method and a control sequence determination device to determine a magnetic resonance system control sequence. Moreover, the invention concerns a method to operate a magnetic resonance system using such a magnetic resonance system control sequence, as well as a magnetic resonance system with a radio-frequency transmission device, with a gradient system, and a control device designed in order to emit a radio-frequency pulse train to implement a desired measurement on the basis of a predetermined control sequence, and in coordination with this to emit a gradient pulse train.
Description of the Prior Art
In a magnetic resonance (MR) tomography system (shortened to “magnetic resonance system”), the body to be examined is typically exposed to a relatively high basic field magnet field (what is known as the B0 field)—for example 3 or 7 Tesla—with the use of a basic field magnet system. In addition, a magnetic field gradient is applied with the use of a gradient system. Radio-frequency excitation signals (RF signals) are then emitted via a radio-frequency transmission system by suitable antenna devices, which cause nuclear spins of specific atoms or molecules in the subject to be excited to resonance, so as to be flipped (deflected) by a defined flip angle relative to the magnetic field lines of the basic magnetic field. This radio-frequency excitation and the resulting flip angle distribution are also designated in the following as a nuclear magnetization, or “magnetization” for short. Upon relaxation of the nuclear spins, radio-frequency signals (known as magnetic resonance signals) are radiated that are received by means of suitable reception antennas and then are processed further. The desired image data can ultimately be reconstructed from the raw data acquired in such a manner. The emission of the radio-frequency signals (what is known as the B1 field) for nuclear magnetic resonance magnetization most often takes place by using an antenna known as a “whole-body coil” that is permanently arranged in the apparatus, around the measurement space (patient tunnel). The reception of the magnetic resonance signals most often takes place with the use of antennas known as local coils that are positioned more closely to the body of the patient. In principle, however, the reception of magnetic resonance signals with the whole-body coil and/or a transmission of the RF signals with the local coils can also take place.
For a specific measurement, a magnetic resonance system control sequence (“control sequence” for short) is defined within an overall operational procedure for the MR system known as a measurement protocol, which also includes additional control specifications, such as a control sequence typically includes a radio-frequency pulse train to be emitted and a gradient pulse train that is switched (activated) in coordination with the RF pulse train. The gradient pulse train has matching gradient pulses in a volume selection direction, for example slice selection direction or slab selection direction, for example in phase coding direction(s) and in the readout direction, often in the z-direction, y-direction and z-direction. This measurement protocol can be created in advance and be retrieved (from a memory, for example) for a specific measurement and be modified by the operator on site as necessary. During the measurement, the control of the magnetic resonance system then takes place wholly automatically on the basis of this control sequence, wherein the control device of the magnetic resonance system reads out the commands from the measurement protocol and executes them.
To generate the control sequence, usually the individual RF pulse trains (i.e. the RF trajectories) are determined in an optimization method for the individual transmission channels over time, depending on a “transmission k-space trajectory” that is typically predetermined by a measurement protocol, or individually by an operator. The “transmission k-space trajectory” (in the following abbreviated only to “k-space trajectory” or “trajectory”) designates with the locations in k-space into which raw data samples are entered by adjusting the individual gradients at specific times. The memory organized as k-space is the spatial frequency domain, and the trajectory in k-space describe the path k-space that is chronologically traversed given emission of an RF pulse by switching of the gradient pulses. By adjustment of the k-space trajectory, it can thus be determined at which spatial frequencies specific RF energy quantities are deposited.
Currently measured B1 maps (that respectively indicate the spatial B1 field distribution for a defined antenna element) and a B0 map (that represents the off-resonances or deviation of the B0 field from the actual desired homogeneous B0 field with spatial resolution, i.e. the actual sought Larmor frequency) can additionally be taken into account in the optimization method to generate the control sequences. Moreover, for the planning of the RF pulse series the user often provides a target magnetization, for example a desired flip angle distribution. With a suitable RF pulse optimization program, the matching RF pulse series is then calculated so that the target magnetization is achieved. In many cases, this is an optimally homogeneous magnetization in the desired field of view (FoV) to be examined, or the desired region to be excited (FoE, Field of Excitation). In-between it is also possible to selectively excite entire defined regions, for example two-dimensionally within a slice or even three-dimensionally, meaning that a non-homogeneous target magnetization is deliberately sought.
A target function is normally set for the RF pulse optimization method or the RF pulse optimization program that is used for this, in which target function the transverse target magnetization is represented in a linear matrix equation system composed of the spatial coil profiles and the multichannel radio-frequency pulse series; information about the present B0 maps and B1 maps and the k-space trajectory that is used also normally enter into the RF pulse optimization method or program. The matrix used in the target function or in the matrix equation system is also designated as an “A-matrix” (since the symbol “A” is typically used for it) or “system matrix” (since it depends on the spatial coil profiles, and thus on the system that is used). In the optimization method, this equation system can then be solved numerically for a defined, predetermined target magnetization in order to achieve the matching radio-frequency pulse series. One example of this procedure is found in the article “Magnitude Least Square Optimization for Parallel Radio Frequency Excitation Design Demonstrated at 7 Tesla with Eight Channels” by K. Setsompop et al., Magn. Reson. Med. 59: 908 to 915, 2008.
Relatively well-optimized radio-frequency pulse trains for a given transmission k-space trajectory can be determined with this method. However, in practice a problem is that these calculations always assume that the trajectory is ideally implemented precisely as it is mathematically defined in the optimization method. However, it is actually the case that the transmission k-space trajectory can differ significantly from the predetermined trajectory upon execution of the sequence. Typical reasons for this are the inadequacies of the gradient system hardware, for example delays, jitter, discretization errors or additional gradient terms that can occur because of induced eddy currents or other effects such as mixing Maxwell terms of different gradient coils. Due to these deviations, not-insignificant blurring, ghost images or geometric distortions of the achieved magnetization or the generated image data can occur between the ideal trajectory curve assumed during the RF pulse optimization method and the trajectory curve that is actually present upon the later emission of the radio-frequency pulse train.
In order to take such deviations of the trajectories or gradients into account, relatively complicated methods could in principle be implemented. For example, in a two-stage method the k-space trajectory is initially traversed once with the predetermined activation data, and the k-space trajectory that is actually achieved are thereby measured. This measured k-space trajectory can then be used within the RF pulse optimization method. The current measurement data could also be used in order to determine error models for the gradient errors. However, all of these methods require a prior measurement of the actual emitted trajectories, which is very time-consuming.